Terahertz spectrometer

ABSTRACT

A terahertz spectrometer includes: a terahertz-wave generating element; an elliptical mirror; an optical lens configured to apply a terahertz wave generated by the terahertz-wave generating element, obliquely to a focusing plane of a first focus of the elliptical mirror; and a terahertz-wave detecting element arranged at a second focus of the elliptical mirror.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent ApplicationJP2008-053802 filed in the Japanese Patent Office on Mar. 4, 2008, theentire contents of which is being incorporated herein by reference.

BACKGROUND

The present application relates to a technique of using electromagneticwaves in a band of about 0.1×10¹² THz to 100×10¹² THz.

A known technique of generating or detecting terahertz waves isterahertz time-domain spectroscopy (THz-TDS). As known in the art, theterahertz time-domain spectroscopy is suitable for use in imagingsamples because the THz-TDS utilizes terahertz waves that defineultra-short pulses, as short as about 100 femtoseconds. Therefore, theterahertz time-domain spectroscopy attracts attention in varioustechnical fields such as industry, medical care, biotechnology,agriculture and security.

In the terahertz time-domain spectroscopy, a pulse light beam emittedfrom an ultra-short laser source is split into a pump beam and a probebeam. The pump beam is focused on a terahertz-wave generating element.In the terahertz-wave generating element, a current flow or electricalpolarization lasts for about subpico seconds, generating a terahertzwave having field amplitude proportional to the time derivative. Theterahertz wave is focused by an optical system on a terahertz-wavedetecting element. At this point, the probe beam is applied to theterahertz-wave detecting element. Then, the terahertz-wave detectingelement generates a carrier. The carrier is accelerated by theelectrical field of the terahertz wave and changed to an electricsignal. The time at which the probe beam reaches the terahertz-wavedetecting element is delayed, thereby measuring the time waveform theterahertz wave has in the amplitude electric field. The time waveform isFourier-transformed, thereby determining the spectrum of the terahertzwave.

In an apparatus that performs the terahertz time-domain spectroscopy, anoptical system guides, to the sample, the terahertz wave generated inthe terahertz-wave generating element, and guides, to the terahertz-wavedetecting element, the terahertz wave generated in the sample. In mostcases, as shown in FIG. 1, this optical system has four parabolicmirrors PM1 to PM4.

This optical system is a transmission type that detects the terahertzwave that has passed though the sample. A reflection-type optical systemis available, which detects the terahertz wave reflected by the sample(see, for example, Jpn. Pat. Appln. Laid-Open Publication No.2004-191302). The parabolic mirrors may be replaced by small-apertureoptical lenses, rendering the lens system small (see, for example, PCTNational Publication No. 2003-518617).

SUMMARY

If the optical system incorporates small-aperture lenses, however, theterahertz wave emanating from the sample will travel off the lensdiameter. In this case, the terahertz wave will not be guided to theterahertz-wave detecting element if the angle of incidence of theterahertz wave applied through the lenses deviates even a little. It isnot known whether this problem is due to a change in the refractiveindex of the sample or to the shape of the sample. The value measured ofthe sample is inevitably poor in reliability.

In consideration of the foregoing, the present application has beenmade. It is desirable to propose a terahertz spectrometer that canmeasure samples at higher accuracy than before possible.

In an embodiment, a terahertz spectrometer includes a terahertz-wavegenerating element, an elliptical mirror, an optical lens, and aterahertz-wave detecting element. The optical lens applies a terahertzwave generated by the terahertz-wave generating element, obliquely tothe first focusing plane of the elliptical mirror. The terahertz-wavedetecting element is arranged at second focus of the elliptical mirror.

The present application in an embodiment can provide a terahertzspectrometer in which a terahertz wave is applied obliquely to anobject, such as a sample, if the object is arranged in the focusingplane having the first focus of the elliptical mirror. The terahertzwave is reflected by the object and guided by the elliptical mirror tothe terahertz-wave detecting element. Hence, the terahertz-wavedetecting element can detect the terahertz wave reflected by the objecteven if the transmission path of the terahertz wave reflected by theobject changes because the object inclines or because the refectionsurface of the object has depressions and projections. The terahertzspectrometer according to the present invention can therefore measure asample at an increased accuracy.

The nature, principle and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by like reference numerals or characters.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram explaining a conventional optical system for guidinga terahertz wave to a sample;

FIG. 2 is a diagram showing the overall configuration of a terahertzspectrometer according to an embodiment;

FIG. 3 is a schematic sectional view showing the configuration of theterahertz-wave transmitting unit of the terahertz spectrometer;

FIG. 4 is a diagram explaining how a beam travels if the reflectionsurface of the object is parallel to the focusing plane having the firstfocus;

FIG. 5 is a diagram explaining how the beam travels if the reflectionsurface of the object inclines to the focusing plane having the firstfocus;

FIG. 6 is a block diagram illustrating the configuration of a computer;and

FIG. 7 is a schematic top view illustrating the configuration of theterahertz-wave transmitting unit of another embodiment.

DETAILED DESCRIPTION

An embodiment will be described with reference to the accompanyingdrawings.

(1) Overall Configuration of the Terahertz Spectrometer

FIG. 2 shows the overall configuration of a terahertz spectrometer 10according to an embodiment of the present invention. The terahertzspectrometer 10 includes an ultra-short laser 11, a dispersioncompensation unit 12, a beam splitter 13, a terahertz-wave generatingelement 14, a time delay element 15, a terahertz-wave detecting element16, and a computer 17.

The ultra-short laser 11 emits a pulse beam having, for example, a pulsewidth of about 100 fs, a pulse repetition frequency of about 80 MHz anda central wavelength of about 780 nm. An ultra-short laser source LS is,in practice, a titanium laser or a sapphire laser, emitting femtosecondpulses.

The dispersion compensation unit 12 adjusts the pulse width of the pulsebeam coming from the ultra-short laser 11, in a specific direction toachieve inverse correction of the pulse width spread that has resultedfrom the wavelength-dependency of optical fibers BF1 and BF2.

The beam splitter 13 splits the pulse beam coming from the dispersioncompensation unit 12 into an excited pulse beam and a detected pulsebeam. The excited pulse beam is guided by a focusing optical unit OU1 tothe optical fiber BF1 and thence to the terahertz-wave generatingelement 14 provided in a terahertz-wave transmission unit TMB.Meanwhile, the detected pulse beam passes through the time delay element15, is then guided by a focusing optical unit OU2 to the optical fiberBF2 and guided through the optical fiber BF2 to the terahertz-wavedetecting element 16 provided in the terahertz-wave transmission unitTMB.

The terahertz-wave generating element 14 generates a terahertz wave thathas field amplitude proportional to the time derivative of the excitedpulse beam. The terahertz-wave generating element 14 is, for example, aphotoconductive antenna or a nonlinear optical crystal such as ZnTe.Note that the photoconductive antenna includes a semiconductor substrateof semi-insulating GaAs or the like, a pair of electrodes formed on thesemiconductor substrate and a voltage applying unit for applying a biasvoltage between the electrodes.

The time delay element 15 delays the time at which the detected pulsebeam reaches the terahertz-wave detecting element 16. In thisembodiment, the time delay element 15 has a pair of mirrors 15 a and 15b. One or both of the mirrors 15 a and 15 b are arranged movable. Theoptical path that extends between the mirrors 15 a and 15 b cantherefore be changed in length to adjust the time at which the detectedpulse beam reaches the terahertz-wave detecting element 16.

The terahertz-wave detecting element 16 detects the terahertz wavegenerated in terahertz-wave generating element 14 and guided through theterahertz-wave transmission path provided in terahertz-wave transmissionunit TMB. The terahertz-wave detecting element 16 generates an electricfield equivalent to the terahertz wave supplied to the element 16through the terahertz-wave transmission path provided in theterahertz-wave transmission unit TMB. When the terahertz-wave detectingelement 16 receives the detected pulse beam from the time delay element15, the terahertz-wave detecting element 16 generates a signal that hasa time waveform of the electric field intensity equivalent to theterahertz wave. Like the terahertz-wave generating element 14, theterahertz-wave detecting element 16 is, for example, a photoconductiveantenna or a nonlinear optical crystal such as ZnTe.

The computer 17 is configured to acquire information from the signalgenerated by the terahertz-wave detecting element 16. The informationrepresents the condition of the sample arranged in the transmission pathprovided in the terahertz-wave transmission unit TMB. Further, thecomputer 17 is configured to appropriately control the time delayelement 15 and the terahertz-wave transmission unit TMB.

(2) Configuration of the Terahertz-Wave Transmission Unit

The configuration of the terahertz-wave transmission unit TMB will bedescribed with reference to FIG. 3. The terahertz-wave transmission unitTMB has a base plate 21. On the base plate 21, a movable stage 22 isarranged. The movable stage 22 can move in a horizontal plane (in x-axisdirection and y-axis direction) parallel to the base plate 12 and in thevertical direction (z-axis direction) perpendicular to the base plate12.

That part of the movable stage 22, which does not face the base plate 21has a passage port 23 for the terahertz wave. A mount plate is providedbelow the passage port 23. Placed on the mount plate is an object RB(i.e., sample SPL or measurement reference of the sample (control)) tobe irradiated with the terahertz wave.

On the back of the movable stage 22, a distance sensor 24 is provided todetect the distance from the object RB. A shield 25 is provided,extending over the front of the movable stage 22 and, of course, thepassage port 23. A shielded space is provided between the inner surfaceof the shield 25 and the front of the movable stage 22. In the shieldedspace, the terahertz-wave generating element 14, the terahertz-wavedetecting element 16, three optical lenses, i.e., aspheric lens 31,elliptical mirror 32 and semispherical lens 33, and a charge coupleddevice (CCD) 34 are arranged, able to move with respect to the movablestate 22, while maintaining a specific positional relationship.

The aspheric lens 31 is made of silicon material, either single crystalor polycrystal. The aspheric lens 31 is provided on that surface of theterahertz-wave generating element 14, from which the terahertz waveemanates. The aspheric lens 31 guides the terahertz wave diverging fromthe terahertz-wave generating element 14, obliquely to the plane inwhich the first focus of the elliptical mirror 32 lies.

The object RB may be placed on the mount plate lying below the passageport 23. In this case, the computer 17 adjusts the height of the movablestage 22 so that the surface having the first focus of the ellipticalmirror 32 and the reflection surface of the object RB (the surface ofthe sample SPL or control, or the deep layer of the sample) may lie inthe same plane.

The elliptical mirror 32 focuses the terahertz wave reflected from theobject RB located on the surface having the first focus, at thedetection surface of the terahertz-wave detecting element 16. In thisembodiment, the elliptical mirror 32 is arranged to apply the terahertzwave reflected by the object RB, back toward a position where theterahertz wave has been emitted.

More specifically, the components are arranged in two areas AR1 and AR2demarked by a plane having a normal NL to the focusing plane FF having afirst focus F1, as shown in FIG. 4 wherein the component identical tothose shown in FIG. 3 are designated by the same reference numbers. Thatis, the terahertz-wave generating element 14, aspheric lens 31,terahertz-wave detecting element 16 and semispherical lens 33 arearranged in the area AR1, and the elliptical mirror 32 is arranged inthe area AR2. More precisely, the elliptical mirror 32 is arranged toguide the terahertz wave, which has been reflected by the object RB,back to the terahertz-wave detecting element 16 provided on the movablestage 22 that lies above the focus plane FF.

The reflection surface of the object RB may be parallel to the focusingplane having the first focus. In this case, as shown in FIG. 4, anincidence angle θ1 and a reflection angle θ2 are equal in view of thelaw of reflection. The incidence angle θ1 is defined by the axis ILA(axis of incidence) of the terahertz wave irradiated from the asphericlens 31 and the normal NL passing the first focus F1 in the focusingplane FF (i.e., reflection surface of the object RB). The reflectionangle θ2 is defined by the normal NL and the axis RLA (axis ofreflection) of the terahertz wave reflected by the focusing plane FF(i.e., reflection surface of the object RB). Thus, the terahertz wavereflected by the reflection surface of the object RB is reflected in thecenter area of the reflection surface of the elliptical mirror 32 andeventually focused on the detection surface of the terahertz-wavedetecting element 16.

On the other hand, the reflection surface of the object RB may inclineto the focusing plane FF. In this case, as shown in FIG. 5, theincidence angle θ1 at which the terahertz wave is applied from theaspheric lens 31 and the reflection angle θ2 at which the terahertz waveis reflected by the reflection surface of the object RB are differentfrom each other. Therefore, the terahertz wave reflected by thereflection surface of the object RB is reflected by the peripheral areaof the reflection surface of the elliptical mirror 32 and eventuallyfocused on the detection surface of the terahertz-wave detecting element16.

Since the object RB inclines to the focusing plane having a first focusor the refection surface of the object RB has depressions andprojections, the elliptical mirror 32 is designed to focus the terahertzwave reflected by the object RB, on the detection surface of theterahertz-wave detecting element 16, even if the transmission path ofthe terahertz wave reflected by the object RB changes.

The semispherical lens 33 is made of silicon material, either singlecrystal or polycrystal. The lens 33 is provided on the detection surfaceof the terahertz-wave detecting element 16. The semispherical lens 33suppresses the reflection of the terahertz wave at the terahertz-wavedetecting element 16.

(3) Configuration of the Computer

The configuration of the computer 17 will be described. As shown in FIG.6, the computer 17 has a central processing unit (CPU) 40, a read onlymemory (ROM) 41, a random access memory (RAM) 42, an operation unit 43,a storage unit 44,a display unit 45, an interface 46, and a bus 47. Thebus 47 connects the ROM 41, RAM 42, operation unit 43, storage unit 44,display unit 45 and interface 46 to the CPU 40. The ROM 41 storesprograms and preset information items. The RAM 42 is used as work memoryfor the CPU 40. To the interface 46, at least the time delay element 15(FIG. 2) and the movable stage 22 (FIG. 3) are connected.

The CPU 40 reads the programs from the ROM 41 and expands them in theRAM 42. In accordance with the programs, the CPU 40 controls, as needed,the storage unit 44, display unit 45 and the interface 46, which performvarious processes.

The CPU 40, which has thus expanded the programs in the RAM 42, isconsidered to have a stage adjusting unit 51, a sample measuring unit52, an image processing unit 53 and a time-delay adjusting unit 54 asillustrated in FIG. 6.

The stage adjusting unit 51 can move the movable stage 22 in accordancewith a distance signal generated by the distance sensor 24 (FIG. 3),setting the movable stage 22 at a prescribed height with respect to theobject RB and holding the focusing plane having a first focus of theelliptical mirror 32 and the reflection surface of the object RB in thesame plane.

Further, the stage adjusting unit 51 can move the movable stage 22 at apredetermined speed in a predetermined scanning direction, i.e., eitherthe x-axis direction or the y-axis direction, thereby to change theposition (focusing position), where the terahertz wave coming from theaspheric lens 31 should be focused on the focusing plane having a firstfocus of the elliptical mirror 32.

Signals generated by the terahertz-wave detecting element 16 are inputto the sample measuring unit 52. More precisely, the sample measuringunit 52 receives two signals (hereinafter called “first detectionsignal” and “second detection signal”). The first detection signal isinput from the terahertz-wave detecting element 16 while the object RBused as a sample SPL, remains arranged on the mount plate lying belowthe passage port 23. The second detection signal is input from theterahertz-wave detecting element 16 while the object RB used as acontrol remains arranged on the mount plate lying below the passage port23. Note that the second detection signal may be stored in the storageunit 44 beforehand and may be acquired from the storage unit 44 whenevernecessary.

After receiving the first and second detection signals, the samplemeasuring unit 52 extracts the amplitude data item and phase data itemabout the terahertz wave from the first signal, and the amplitude dataitem and phase data item about the terahertz wave from the secondsignal. From the difference between the amplitude data items and thedifference between the phase data items, the sample measuring unit 52generates information that represents the condition of the sample SPL.

On receiving the information representing the condition of the sampleSPL, the image processing unit 53 causes the display unit 45 to displaythe condition of the sample SPL, which is represented by theinformation. The user can therefore visually recognize the condition ofthe sample SPL.

When the user operates the operation unit 43, instructing the displayunit 45 to display the condition of the sample SPL, the image processingunit 53 causes the display unit 45 to display the image represented bythe image signal generated by the CCD 34. The user can thereforevisually recognize the passage port 23 and the object RB arranged belowthe passage port 23.

The time-delay adjusting unit 54 is configured to determine a desiredlength of the optical path that extends between the mirrors 15 a and 15b of the time delay element 15, from the signal input at the operationunit 43 (not shown), and to move one or both of the mirrors 15 a and 15b, thereby to set the length of the optical path to the desired value.

(4) Operation and Effect

In the terahertz spectrometer 10 configured as described above, theterahertz-wave detecting element 16 is arranged at the second focus ofthe elliptical mirror 32, and the terahertz wave generated by theterahertz-wave generating element 14 is applied obliquely to the planein which the first focus of the elliptical mirror 32 lies.

In the terahertz spectrometer 10, the terahertz-wave detecting element16 can therefore detect the terahertz wave applied obliquely to theobject RB and reflected by the object RB if the object RB is placed inthe focusing plane FF (see FIG. 3 and FIG. 4).

Further, in the terahertz spectrometer 10, the terahertz wave reflectedby the focusing plane FF having a first focus F1 of the ellipticalmirror 32 is guided to the terahertz-wave detecting element 16 arrangedat the second focus of the elliptical mirror 32. The terahertz wavereflected by the object RB can be focused on the detection surface ofthe terahertz-wave detecting element 16 even if the transmission path ofthe terahertz wave reflected by the object RB changes because the objectRB inclines with respect to the focusing plane FF or because thereflection surface of the object RB has depressions and projections (seeFIG. 5).

In the terahertz spectrometer 10, the terahertz-wave generating element14, aspheric lens 31, terahertz-wave detecting element 16 andsemispherical lens 33 are arranged in the area AR1, and the ellipticalmirror 32 is arranged in the area AR2, the areas AR1 and AR2 beingdemarked by a plane having a normal NL to the focusing plane FF havingthe first focus F1 (see FIG. 4 and FIG. 5).

Thus, in the terahertz spectrometer 10, the terahertz wave applied fromthe area AR1, reflected from the object RB and travelling toward thearea AR2 can be applied back toward a position where the terahertz wavehas been emitted. The terahertz-wave generating element 14 and theterahertz-wave detecting element 16 can therefore be arranged close toeach other. This helps to miniaturize the terahertz spectrometer 10.Moreover, the terahertz-wave generating element 14 and theterahertz-wave detecting element 16 can be closer to each other than inthe case where the terahertz wave applied to the object is applied backafter passing through the object, even if no optical system is used tochange the optical path with respect to the transmission path betweenthe elliptical mirror 32 and the terahertz-wave detecting element 16.

Further, in the terahertz spectrometer 10, the terahertz-wave generatingelement 14, aspheric lens 31, terahertz-wave detecting element 16 andsemispherical lens 33, all provided in the area AR1, are arranged abovethe focusing plane FF having the first focus F1 (see FIG. 4 and FIG. 5).

This means that the terahertz-wave generating element 14 and theterahertz-wave detecting element 16 can be arranged even closer to oneanother. The terahertz spectrometer 10 can therefore be made muchsmaller. For example, the terahertz-wave generating element 14 and theterahertz-wave detecting element 16 may be arrayed to provide a probe.Further, the movable stage 22 can be arranged to avoid the interferenceof the terahertz wave, more easily than in the case where, in the areaAR1, the terahertz-wave generating element 14 and terahertz-wavedetecting element 16 are arranged, respectively, above and below thefocusing plane FF having the first focus F1.

In the terahertz spectrometer 10, the aspheric lens 31, which is made ofsilicon material, is used as an optical lens (see FIGS. 3 to 5). Theaspheric lens 31 can therefore be adjusted to the terahertz-wavegenerating element 14 in terms of refractive index. This can reduce theenergy loss in the terahertz wave generated by the terahertz-wavegenerating element 14 and applied to the object RB, which results fromthe reflection of the terahertz wave at the optical lens. In addition,since a single lens can alone suppress the aberration, the number oflenses used and the sizes thereof are smaller than in the case where aspherical lens or a parabolic mirror is used. This serves to miniaturizethe terahertz spectrometer 10, too. No anti-reflection film is bonded tooptical lenses for guiding terahertz waves in most cases to reduce theinfluence of multipath reflection, though anti-reflection film is bondedto the optical lenses for ordinary uses. In view of this, the use of asingle lens is particularly useful.

Further, in the terahertz spectrometer 10, the optically system providedbetween the terahertz-wave generating element 14 (terahertz-wavedetecting element 16) and the sample SPL is composed of a singleaspheric lens 31 (semispherical lens 33). This more reduces the numberof interfaces in which unnecessary reflection may occur than in the casewhere the optical system is composed of a plurality of lenses. Theterahertz wave can therefore be improved in intensity. Ultimately, themeasuring accuracy can be enhanced.

In the terahertz spectrometer 10, the computer 17 moves the movablestage 22 in accordance with the distance detected by the distance sensor24, i.e., the distance from the sample SPL placed at the first focus F1.The focusing plane FF having the first focus F1 is thereby held in flushwith the reflection surface of the sample SPL (see FIG. 3 and FIG. 6).

In the terahertz spectrometer 10, the terahertz wave reflected by thesample SPL (i.e., object used as a sample or a control) can be guided tothe terahertz-wave detecting element 16, irrespective of the size orshape of the sample SPL. Thus, various kinds of samples can be measuredwith high accuracy.

In the terahertz spectrometer 10, the computer 17 moves the movablestage 22, changing the position at which the terahertz wave coming fromthe aspheric lens 31 is applied, with respect to the focusing plane FFhaving the first focus F1. The information about the position where theterahertz wave is applied to the sample SPL placed in the focusing planeFF is acquired from the signal generated by the terahertz-wave detectingelement 16 (see FIG. 3 and FIG. 6).

The terahertz spectrometer 10 can therefore perform the imaging of agreater part of the sample, enhancing the measuring accuracy. Theterahertz spectrometer 10 is advantageous over the Fourier spectroscopyin that the terahertz spectrometer 10 achieves a high S/N ratio and canacquire amplitude data and phase data at the same time.

In the configuration described above, the terahertz wave is appliedobliquely to the sample SPL arranged in the focusing plane FF having thefirst focus F1 of the elliptical mirror 32. The sample SPL reflects theterahertz wave, which is detected by the terahertz-wave detectingelement 16 provided at the second focus F2 of the elliptical mirror 32.The terahertz wave reflected by the sample SPL can therefore be focusedon the detection surface of the terahertz-wave detecting element 16 evenif the transmission path of the terahertz wave reflected by the sampleSPL changes because the sample SPL inclines with respect to the focusingplane FF of the first focus or because the refection surface of thesample SPL has depressions and projections. Thus, the terahertzspectrometer 10 can measure the sample at an increased accuracy.

(5) Other Embodiments

The embodiment described above has an aspheric lens 31. This inventionis not limit to this, nevertheless. For example, a refractive lenscomposed of N lenses arranged in a line or such parabolic mirrors asshown in FIG. 1 may be used. That is, one or more optical lenses of anytype can be used if they can apply the terahertz wave generated by theterahertz-wave generating element 14, obliquely to the focusing plane FFhaving the first focus F1 of the elliptical mirror 32. As specifiedabove, the aspheric lens 31 is made of silicon material. Instead, theaspheric lens 31 may be made of any other material as long as theaspheric lens 31 can be adjusted to the terahertz-wave generatingelement 14 in terms of refractive index.

In the embodiment described above, one terahertz-wave generating element14 and one aspheric lens 31 are arranged in the terahertz-wavetransmission unit TMB. This invention is not limited to thisconfiguration. Two or more terahertz-wave generating elements 14 and twoor more aspheric lenses 31 may be arranged in the terahertz-wavetransmission unit TMB.

FIG. 7 shows a terahertz-wave transmission unit having twoterahertz-wave generating elements and two aspheric lenses. In FIG. 7,the components identical to those shown in FIG. 3 are designated by thesame reference numbers. FIG. 7 shows the components as viewed from theshield 25 (FIG. 3) toward the movable stage 22.

In this terahertz-wave transmission unit TMB, two excited pulse beamsare applied to two terahertz-wave generating elements 14A and 14Bthrough optical fibers BF1 a and BF1 b, respectively. The optical fibersBF1 a and BF1 b are independent fibers or fibers branched from onefiber. The terahertz-wave transmission unit TMB focuses the terahertzwaves applied from the terahertz-wave generating elements 14A and 14Bthrough the aspheric lenses 31A and 31B, respectively, to a sample SPL.The sample SPL reflects the terahertz waves. The terahertz waves thusreflected are reflected by an elliptical mirror 32. A semispherical lens33 focuses the terahertz waves on a terahertz-wave detecting element 16.

One terahertz-wave detecting element is provided for all terahertz-wavegenerating elements in the terahertz-wave transmission unit TMB of FIG.7. The terahertz-wave transmission unit TMB can therefore be smallerthan in the case where terahertz-wave detecting elements are providedfor the terahertz-wave generating elements, respectively.

In the terahertz-wave transmission unit TMB of FIG. 7, the opticallenses (i.e., elliptical mirror 32 and semispherical lens 33) are usedto guide both terahertz waves from the sample SPL to the terahertz-wavedetecting element 16. The unit TMB can therefore be much smaller than inthe case two elliptical mirrores and two semispherical lenses are usedto guide the two terahertz waves, respectively.

Further, as shown in FIG. 7, the incidence angles at which the twoterahertz waves are applied to the focusing plane FF having the firstfocus F1 of the elliptical mirror 32 are equal or similar. Hence, if theaspheric lenses 31A and 31B have different numerical apertures, the spotplane each terahertz wave has with respect to the sample SPL can bealtered merely by using a switching unit for switching theterahertz-wave generating element, from the element 14A to the element14B, or vice versa. Therefore, information items representing differentresolutions or different parts of the sample can be acquired for thesample SPL. This helps to increase the measuring accuracy. The switchingunit may be one that switches the wavelength or pulse width of theexcited pulse beam applied to the terahertz-wave generating elements 14Aand 14B, or switches the element to which the excited pulse beam isapplied, or switches not only the wavelength or pulse width, but alsothe element.

The aspheric lenses 31A and 31B may have different incidence angles withrespect to the first focus F1 of the elliptical mirror 32, though theyhave the same numerical aperture or similar apertures. In addition, aswitching unit may be used to switch the terahertz-wave generatingelement, from the element 14A to the element 14B, or vice versa. If thisis the case, the depth to which the terahertz waves reach in the sampleSPL can be changed to acquire accurate information about the layerstructure of the sample. This will increase the measuring accuracy.

The aspheric lenses 31A and 31B may have the same incidence angle withrespect to the first focus F1 of the elliptical mirror 32, and may havethe same numerical aperture or similar numerical apertures. Using theseaspheric lenses 31A and 31B, the terahertz waves may be focused at thesame position. Then, the sample SPL will be irradiated with a terahertzwave that has an increased intensity (brightness). As a result, themeasuring accuracy will be higher, and the components that cooperate togenerate terahertz waves can be more durable, than in the case whereonly one terahertz-wave generating element is used.

Moreover, the various designs specified above can be combined in anypossible way.

In the embodiment described above, the movable stage 22 is moved, movingthe optical lenses 31 to 33, terahertz-wave generating element 14 andterahertz-wave detecting element 16 without changing the positionalrelationship of these components, thereby to hold the focusing plane FFhaving the first focus F1 in flush with the reflection surface of thesample SPL. This invention is not limited to this, nevertheless.Instead, the mount plate holding the sample SPL may be moved to hold thefocusing plane FF having the first focus F1 in flush with the reflectionsurface of the sample SPL.

In the embodiment described above, the movable stage 22 is moved, movingthe optical lenses 31 to 33, terahertz-wave generating element 14 andterahertz-wave detecting element 16 without changing the positionalrelationship of these components, thereby to change the position wherethe terahertz wave coming from the aspheric lens 31 is applied to thefocusing plane FF having the first focus F1. This invention is notlimited to this, nevertheless. Instead, the mount plate holding thesample SPL may be moved to change the position where the terahertz wavecoming from the aspheric lens 31 is applied to the focusing plane FFhaving the first focus F1.

The present application can be utilized in various technical fields suchas industry, medical care, biotechnology, agriculture, security, datacommunication and electronics.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. A terahertz spectrometer comprising: a terahertz-wave generatingelement; an elliptical mirror; an optical lens configured to apply aterahertz wave generated by the terahertz-wave generating element,obliquely to a focusing plane of a first focus of the elliptical mirror;and a terahertz-wave detecting element arranged at a second focus of theelliptical mirror.
 2. The terahertz spectrometer according to claim 1,wherein the terahertz-wave generating element, the optical lens, and theterahertz-wave detecting element are arranged in a first area, and theelliptical mirror is arranged in a second area, the first and secondareas being demarked by a plane having a normal to the focusing planepassing through the first focus.
 3. The terahertz spectrometer accordingto claim 2, wherein the terahertz-wave generating element, the opticallens, and the terahertz-wave detecting element are arranged above thefocusing plane located in the first area.
 4. The terahertz spectrometeraccording to claim 1, wherein the optical lens is an aspheric lens madeof silicon material.
 5. The terahertz spectrometer according to claim 1,wherein the terahertz-wave generating element is provided in plurality,and the optical lens is provided in plurality, each optical lens beingan aspheric lens made of silicon material and arranged in associationwith one terahertz-wave generating element.
 6. The terahertzspectrometer according to claim 5, wherein the aspheric lenses arrangedin association with the terahertz-wave generating element have differentnumerical apertures.
 7. The terahertz spectrometer according to claim 5,wherein the aspheric lenses arranged in association with theterahertz-wave generating element receive terahertz waves at differentincidence angles with respect to the focusing plane of the first focusof the elliptical mirror.
 8. The terahertz spectrometer according toclaim 6 or 7, further comprising a switching unit for switching theterahertz wave generated by one terahertz-wave generating element to theterahertz wave generated by another terahertz-wave generating element.9. The terahertz spectrometer according to claim 1, further comprising:a sensor configured to detect a distance from an object arranged at thefirst focus; and a holding unit configured to hold the focusing planeand a reflection surface of the object on the same level based on thedistance detected by the sensor.
 10. The terahertz spectrometeraccording to claim 1, further comprising: a changing unit configured tochange the position in the focusing plane, where the terahertz wavecoming from the optical lens is applied to the focusing plane; and anacquiring unit configured to acquire information about the position ofthe object arranged in the focusing plane, using a signal detected bythe terahertz-wave detecting element.